1. Field of the invention
The present invention relates to a method of controlling clamping and unclamping of a hydraulic chuck for use on machine tools on which the workpiece must be rotated in order to machine it.
2. Description of the prior art
Chucks of this type are well known in the art and one such chuck is described by assignees of the applicants in U.S. Pat. No. 5,292,139.
They are generally operated by a double-acting actuator mounted on the rotating shaft of the machine tool, actuating the jaws in a clamping or unclamping direction and coupled to a hydraulic control system controlled by a control loop. A system of this kind enables the clamping force on the workpiece to be modified during machining, for example to change from blank machining to finish machining.
The skilled person knows that blank machining, which is generally required to be completed as quickly as possible, requires a high clamping force at the chuck which cannot be maintained during finish machining, not least to prevent deformation of the workpiece, for example when it is hollow and the clamping is external to the workpiece.
Until now chucks have used an open loop to control the pressure in accordance with a pressure set point specified by the operator. However, an open loop of this kind cannot automatically track changes in the pressure and provide reliable information on correct execution of an instruction to change pressure. This goes against the need to achieve safe operation under all circumstances.
In the control engineering field, correct execution of instructions to change a control magnitude can be achieved using a closed loop comprising a feedback channel that is looped to the loop input, to which the set point signal and a signal measuring the real output are also applied.
However, the idea of applying closed loop control to a chuck of the above kind runs up against the specific problem of the presence of safety valves in the chuck. For obvious safety reasons the chuck is provided with respective safety valves in the feed pipes immediately upstream of the two chambers of the actuator to maintain the pressure therein if the feed pressure drops quickly and by a large amount, or is even lost altogether, because of any kind of operating incident. Without this precaution, the workpiece to be machined might escape from the chuck in the event of an incident.
In the event of a command to reverse the direction of the double-acting clamping actuator to clamp or unclamp the workpiece, the pressure in the chambers of the actuator varies suddenly from a relatively high value to zero, or vice versa. Under such conditions, a closed loop control system cannot establish the new pressure without a transient period, in which the pressure can take values that are difficult to control, before the loop stabilizes on the value corresponding to the new pressure in the corresponding chamber of the actuator. Any such sudden variation during the transient period may cause a high pressure that is maintained by the safety valves. The latter therefore maintain the pressure established beforehand by isolating the actuator from the hydraulic control system, which clamps up the chuck, although in reality there is no condition present necessitating operation of the safety valves.
An object of the invention is to provide a method of controlling a hydraulic chuck for rotary machine tools which does not have the drawbacks mentioned above and enables closed loop control to be used in the control system.